Protection circuits

ABSTRACT

A protection circuit suitable for use with electronic test equipment contains a switch which is open circuited if an overload condition is detected. The opening of the switch isolates the overload detector from the applied overload thereby allowing the detector to have a relatively low power handling capability. If the switch is manually closed whilst the overload condition persists the switch is automatically re-opened within a short predetermined time.

This invention relates to protection circuits which are suitable for usewith certain kinds of electronic equipment which receive signals ofvarying frequency having wide variations in amplitude level. Commonlythe measuring circuits themselves within the electronic equipment arecapable of handling signals having only a low dynamic range and they canbe damaged by the application of signals having excessive amplitudelevels. Whilst it is desirable to protect the equipment against theapplication of such excessive amplitude levels, it is neverthelessundesirable to restrict the application of a signal which is withinacceptable limits and the present invention seeks to provide an improvedprotection circuit.

According to this invention a protection circuit includes a switchpositioned between a radio frequency detector and an input terminalwhich in operation receives an applied radio frequency signal; means foropening said switch when an output signal provided by said radiofrequency detector exceeds a predetermined level which is representativeof an overload condition; means for latching the switch in thiscondition manually operable re-set means for closing said switch; andmeans for re-opening said switch within a predetermined time duration ofsaid switch being closed if the overload condition persists.

Preferably means are provided for preventing repeated operation of saidswitch if the manually operable re-set means is maintained in the re-setcondition whilst the overload condition persists.

Preferably said switch is constituted by an electromagnetically operatedrelay which when not energised, holds said switch open.

Preferably again a visual indication is provided of the conductive stateof said relay.

Preferably the protection circuit also includes at least one switchableattenuator which follows the switch. This allows the dynamic range ofthe electronic equipment with which the protection circuit is used to beincreased. The attenuator is subject to an overload condition onlywhilst said switch is closed, and said predetermined time duration ischosen so that it is sufficiently short to avoid permanently damagingthe attenuator.

Preferably again a plurality of switchable attenuators are provided inseries, the individual attenuators being progressively brought into orout of circuit in dependence on changes in the level of the appliedsignal.

Preferably yet again means are provided for ensuring that when saidattenuators are switched into or out of circuit the resulting change insignal level does not cause the attenuators to be switched out of orinto circuit respectively until a predetermined change occurs in thelevel of the applied radio frequency signal at said input terminal.

The invention is further described by way of example with reference tothe accompanying drawing which illustrates a protection circuit inaccordance with the invention.

Referring to the drawing, a protection circuit is shown which issuitable for use with a modulation meter. A radio frequency signal ofvariable amplitude is applied to an input terminal 1 and is passed via aswitch 2, switchable attenuators 3 and 4 and frequency changer 10 to anoutput terminal 5, which in normal use is connected to the modulationmeter. The output of the switch 2 is fed to a radio frequency detector6, which produces from the signal applied to it a d.c. level which iscompared at a comparator 7 with a reference level indicative of anoverload condition. The output of the comparator 7 is fed via a latch 8to control the operation of the switch 2 and an indicator lamp 20.

A frequency changer 10 receives the input signal via switch 2 andattenuators 3 and 4 and produces a signal having a fixed predeterminedfrequency, but having an amplitude which is the same as, or directlyrelated to, the amplitude of the signal applied to it. This fixedfrequency signal is applied to a linear detector 11, which produces ad.c. output level which is closely related to the amplitude of theapplied fixed frequency signal. The output of the detector 11 is appliedto respective inputs of comparators 12, 13, 14 and 15. The comparator 12controls the operation of the switchable attenuator 4 via a latch 16.The comparator 13 controls the operation of the switchable attenuator 3via a latch 17, and the comparators 14 and 15 respectively controlwarning lamps 18, 19 which indicate when the input signal applied toterminal 1 is excessively low or excessively high. In practice the lamps18, 19 and 20 would conveniently be light emitting diodes.

The circuit shown in the drawing operates as follows. When a very lowamplitude signal is applied to input terminal 1, the switch 2 is heldclosed and the two switchable attenuators 3 and 4 are switched so as tobe short circuited so that the signal is passed to the frequency changer10 without attenuation. If the signal level is too low to enable areliable masurement to be made on it by circuits coupled to outputterminal 5, the low level is detected by the comparator 14 and thewarning lamp 18 is energised. A reference low level signal is appliedover line 26 to the second input of the comparator 14. Additionally thecomparator 14 re-sets the latch 16 via line 21 to ensure that theattenuator 4 is not in circuit. As the input level applied to terminal 1increases the comparator 12 detects when a first threshold value isreached and enables latch 16, which switches attenuator 4 into circuit.A suitable reference threshold level is applied over line 27. Theattenuator provides approximately 12 dB attenuation to the input signal.At the same time a signal is sent from latch 16 over line 22 to preventoperation of the attenuator 3 until attenuator 4 has been brought intocircuit. It will be appreciated that when attenuator 4 is in circuit thelevel of the signal obtained at detector 11 and applied to comparators12 and 13 reduces in amplitude but not to the level at which the lowcomparator 14 is triggered. If the signal is still sufficiently great toexceed an initial reference level applied to comparator 13 over line 23and output from this comparator is sent via the latch 17 to bring thesecond attentuator 3 into circuit, so as to provide a further 12 dBattenuation. At the same time the latch 17 adjusts the reference levelapplied to the comparator 13 over line 23 so as to decrease it. This ineffect provides the latching action.

Both of the comparators 12 and 13 are given hysterisis in their inputsignal latch operating levels. That is when their respective attenuators4 or 3 are switched into circuit, thus reducing the signal input levelto the comparator 12 or 13, the comparators do not immediately unlatchthe latches 16 or 17 and a reduction in the signal level at terminal 1must occur before either of the latches 16 or 17 will unlatch and allowthe attenuator 4 or 3 to be short-circuited. When either attenuator isshort-circuited the comparator hysterisis ensures that an increase inthe signal level at terminal 1 must occur before either attenuator isre-inserted.

If the level of the input signal applied to terminal 1 increases furtherto exceed a reference level applied via line 28 the comparator 15 istriggered and a warning lamp 19 is energised, which indicates that anexcessively high input signal is being received which will introducemeasurement inaccuracies in the modulation meter. An interlock signal isreceived from the latch 17 via line 24 to prevent operation of thewarning lamp 19 unless both attenuators 3 and 4 are already in circuit.

If a signal is received at the input terminal 1 which is substantiallygreater than that required to produce illumination of the warning lamp19, the overload condition is detected by the comparator 7, which sets alatch 8, which in turn operates the switch 2 to open it and alsoilluminates an overload warning lamp 20. In practice the switch 2 is amagnetically operated relay, since a relay does not distort the radiofrequency signal passing through it and it is also capable ofwithstanding very high open circuit voltages. When the switch 2 is openthe line connecting it to terminal 1 is in effect an unmatched radiofrequency transmission line and it is possible for very high voltages toappear across the open contacts of the switch 2. Because the switch 2 isa magnetically operated relay, its speed of operation is relatively slowand thus the switch is opened a short predetermined time after theoverload condition is detected. This predetermined delay is such thatthe application for this very short period of the overload signal doesnot seriously overheat or permanently damage the attenuators 3 and 4 orthe frequency changer 10. The latch 8 maintains the switch to open untilit is manually re-set by means of re-set button 9.

The circuitry is arranged so that switch 2 is only in the closedcondition when the circuitry is energised and the signal input toterminal 1 is below the predetermined overload level. This ensures thatthe equipment is protected even when the overload protection circuit isnot switched on.

As the switch 2 is between the input signal to terminal 1 and thedetector 6, the detector is isolated from the input signal source whenthe said switch opens under overload conditions and thus the detector 6does not have to withstand the overload condition and is itselfprotected by the switch 2. As the detector 6 is inoperative when switch2 is open, latch 8 is necessary to maintain the switch 2 in the opencondition until the latch 8 is manually re-set. If the overloadcondition is still present when the latch 8 is re-set the detector 6detects the overload and operates the latch 8 to again open the switch2. The re-set circuit is arranged as a "one shot" system; that is thelatch 8 is only re-set when the re-set 9 is initially operated. Thisprevents the switch 2 repeatedly opening and closing if the re-set isheld closed.

We claim:
 1. An r.f. circuit for increasing the dynamic range of ameasuring instrument while providing protection against overload, saidr.f. circuit having an input terminal which in operation receives anapplied r.f. input signal having wide variations in amplitude level andsaid circuit having an output terminal for connection to the measuringinstrument;said r.f. circuit including a switch connected to said inputterminal and having an output; switchable attenuator means connectedbetween said output of the switch and said output terminal forselectively attenuating the r.f. input when the amplitude level thereofreaches a first predetermined amplitude level and including means fordetecting the presence of said first predetermined amplitude level ofthe input signal; latch means connected to said switch for latching theswitch in open and closed conditions and including manual reset meansfor causing the switch to be latched in the closed condition; and meansconnected to the output of said switch and to said latch means fordetecting the presence of a second predetermined amplitude level of ther.f. input signal and actuating said latch means to cause said switch tobe latched in the open condition, said second predetermined amplitudelevel being greater than said first predetermined level.